``From the outset we have been open to any and all theories, and the investigation continues on any and all theories," he said.
The Postal Service has taken action to try to prevent a repeat.
``We have fully deployed the fleet of biodetection systems" on canceling machines at 271 mail processing locations, Tom Day, postal vice president, said in a telephone interview.
Installation of the current system cost $800 million, provided by Congress, and the Postal Service is spending about $70 million to operate it. That annual cost is expected to climb to $120 million.
The detectors check for anthrax and two other biological hazards, which Day declined to name.
Among those killed in 2001 were two postal workers at Washington's Brentwood mail processing facility. Day said workers now are trained to look for suspicious packages and call in inspectors if they detect something unusual.
Among the things that make a package suspicious are leaking powder and liquids. In addition, there are other telltale signs that the agency does not like to discuss for fear of tipping off terrorists.
Last week, the FBI denied it had overestimated the potency of the anthrax spores used in the killings.
Shortly after the attacks, there were reports that the spores contained additives and had been subjected to sophisticated milling -- both techniques used in anthrax-based weapons -- to make them more lethal. But bureau officials now say the early media reports of weaponized anthrax were misconceptions.
If the anthrax used was less sophisticated than originally thought, that opens up a wider field of potential suspects.
A small number of people in the United States and abroad are being looked at by investigators because they fit some criterion, such as access to anthrax, said one official who declined to be identified because authorities are reluctant to discuss the details of ongoing investigations.
